mixing proceeds rapidly if the mixing proceeds rapidly (t_mix is small), the droplet size...

If the mixing proceeds rapidlymixing proceeds rapidly (T_mix is small), then the fields of water vapor and temperature will be the same for all droplets. Thus, all droplets will experience nearly the same sub-saturation and will shrink toward smaller sizes until equilibrium is reached. Consequently, the droplet size distribution is shifted the droplet size distribution is shifted toward smaller diameters.toward smaller diameters.This type of mixing is termed homogeneousThis type of mixing is termed homogeneous

If, in contrast, the mixing proceeds slowlythe mixing proceeds slowly and the borders betweencloudy and sub-saturated ambient air exist long enough, individual droplets adjacent to the borders will completely evaporate, while other droplets that are surrounded by saturated cloudy air remain unaffected.In this case, the droplet size distribution will not be shifted the droplet size distribution will not be shifted toward smaller sizestoward smaller sizes, but the droplet number will be droplet number will be diminisheddiminished by the complete evaporation of single droplets. This is referred to as inhomogeneous mixing.inhomogeneous mixing.

Number concentration

size

IH: mixing proceeds slowly (mean) size : unchaged #/cc : dec.

H: mixing proceeds fast (mean) size : dec. #/cc : (?)

More moist ambient air:

Drier ambient air entrained and mixing

entrainment of saturated air (S=1) results in dilution and therefore reduction of nd, but no evaporation and decrease in D.

T_mix: mixing time scale

T_react: the time for complete chemical reaction (or phase change)

- related to the time required for phase changes associated with droplet evaporation and the restoration of saturation in the mixture

(1) Damkohler number

Da >>1T_mix > T_react Mixing proceeds slowly inhomogeneous mixing

Da << 1T_mix < T_react Mixing proceeds fast homogeneous mixing

Most often in the cloud physics literature T_react in Eq. (1) is given by the evaporation time for droplets, which for a droplet of diameter D is given by

Fk: thermodynamic terms associated with the heat conduction Fd : ’’ the water vapor diffusion,S : the water vapor saturation ratio.This time scale derives from the droplet condensation growth law

under the assumption of constant thermodynamic conditions(i.e., constant S). However, another time scale

(3)

(2)

However, another time scale exists that characterizes how rapidly an equilibriumvapor saturation is reached by evaporation (or growth) of an entire population of droplets. If we neglect source terms for the super-saturation (i.e., rising air motions)and just consider a droplet population within a subsaturated environment, the equation for the supersaturation can be written as

where s=S-1 is the supersaturation and η ~ (ndD)^-1, with D being the mean diameter of the droplet population (e.g., Rogers and Yau 1989, p. 110).

The supersaturation field responds with an exponential time scale of Tp= η^-1 when D is held constant. This time scale is referred to as the phase-relaxation time (Politovich and Cooper 1988; Cooper 1989).

(4)

During a mixing event, neither assumption is exactly correct. Results of coupling the microphysical and thermodynamic fields [(i.e., allowing the supersaturation and integral radius to change in Eqs. (3) and (4)] are shown in Fig. 2 for idealized, uniform-size droplet populations

if one of the two traditionally defined time scales is much smaller than the other, the time scale of the coupled system is close to the ‘‘dominant’’ time scale. For example: Fig. 2l, Tp (0.43 s) is much smaller than te (5.52 s), and the resulting time scale of the coupled system is 1.50 sif te ~ tp, the resulting time scale can be much larger than the two individual time scales. Ex) Fig 2c.

TpTe

the mixing time scale and therefore the Damkohler number strongly depend on the chosen mixing length scale lE.

where epsilon is the turbulent energy dissipation rate

the classical picture of the turbulent energy cascade, is one of a continuous spectrum of turbulent eddy sizes.

Instead of considering only a single mixing time or length scale, we recognize that there is a continuum of mixing time scales, and likewise a continuous range of Damkohler numbers.

As a result of this continuous range of scales, we alter the question from ‘‘At what scale does mixing occur?’’ to‘‘At what scale does mixing make the transition from inhomogeneous to homogeneous?’’.

The revised question can be answered by settingDa=1 and then calculating the corresponding mixing length scale via

Dry air is entrained into cloud at lE, but during the subsequent mixing a transition from inhomogeneous to homogeneous mixing occurs at l*.a transition from inhomogeneous to homogeneous mixing occurs at l*.

Therefore, after a large blob of size lE of sub-saturated air has been entrained,all filaments of size lE> l> l* will experience inhomogeneous mixing, whereas smaller filaments will mix homogeneously.

(6) Transition length scale

inhomogeneous

homogeneous

Da >>1

T_mix > T_react

Mixing proceeds slowly inhomogeneous mixing

experiment

all measurements were made within ;50–100 m from cloud top- their microphysical characteristics (e.g., mean droplet number and diameter) are similar

a) Time series of droplet number concentration: max~870cm^-3

b) median droplet diameter : ~7microns in more diluted cloud more diluted cloud parts(B)parts(B), 12microns in regions where the #/cc is the largest.

c) Regions where #/cc is the largest : Rising air motion ~ 1.7m/s

Regions with decreased droplet concentration : Descending air w~-1.3m/s

- W is well correlated with T

d) Ascending cloud is a region of intense turbulence compared to the surroundings

10^-2 m^2s^-3 : at the edge of the updraft region

10^-4-10^-3m^2s^-3: in most of the more diluted cloud regions

From Fig. 5a it can be concluded that the mixing is close to the homogeneous scenario in some regions of the cloud.

follows the homogeneous mixing line for entrained air with a saturation ratio between 0.90 and 0.95.

For (Dv /Dv,a)^3 < 0.6, the close correlation between the data and the homogeneous mixing lines is lost, and it is not obvious whether mixing is homogeneous or inhomogeneous.

The PDF is calculated for the areas labeled A and B in Fig. 4a, where B is a region that is more diluted compared to region A.

Clearly, the whole size distribution taken in the more diluted region B is shifted toward smaller sizes.

B: More diluted

CASE 2

For this flight, fast temperature measurements were not available, so the temperature trace is not shown.

Dv and nd are highly correlated(e.g., between 35 600 and 35 700 s UTC).

The mixing diagram is shown

As in case I, the different coloring of a subset of the [(nd/nd,a); (Dv /Dv,a)^3] values is based on the fact that this data stems from a region with different flow properties, further discussed in section 4g.

homogeneous mixing for higher values of (nd/nd,a) (black circles).

inhomogeneous mixing below this value, where the points lie nearly along a horizontal line (gray circles),

The size distribution for themore diluted region (A) : shifted toward smaller droplet sizes compared to the size distribution for region B.

For regions A and B, the altitude is constant within 10 m; therefore, it can be concluded that the shift of the droplet size PDF toward smaller sizes is caused by homogeneous mixing of subsaturated air and subsequentevaporation of droplets.

CASE 3 : About 80 m above the flight leg case I, and about 15 min later

From Fig. 4-III(d) : cloud section is lessturbulent compared to the first two examples (max. ε ~ 10^-3).

There is no significant trend for ε to be higher in cloudy regions than in the ambient dry air, suggesting that the cloud is dissipating.

LWC, w, and T are much less correlated than in case I.

The fact that the mean volume diameter of all samples is much smaller than the adiabatic value suggests that homogeneous mixing acted before to make the clouds as dilute as they are, but further dilution is via inhomogeneous mixing.

Both droplet diameter and concentration are below the adiabatic values for that altitude. (?)

With increasing dilution (toward smaller (nd/nd,a) values), (Dv /Dv,a)^3 decreases only slightly indicates mixing is close to the inhomogeneous scenario.

For the regions where the droplet concentration is diminished (curve B), the mode diameter is only slightly shifted toward smaller diameters compared to the region of relatively high concentration (curve A).

CASE 4

Maximum values of the droplet number concentration up to~600cm^-3 and maximum values of LWC of 0.6 gm^-3

The median droplet diameter wasaround 13 microns.

The vertical velocity reveals sharp fluctuations between +3.1 m/s and -1.4 m/s.

The time series of temperature is hardly correlated with w and LWC, implying that this cloud is aged and not actively growing.

energy dissipation rate shows maxima up to 10^-2 (due to the fluctuating vertical velocity), but, similar to w, ε shows strong fluctuations even within the clouds. In cloud-free regions, ε drops to ~10^-5 m^2 s^-3.

For ratios (nd/nd,a) > 0.6, the data points scatter around a horizontal line; that is, (Dv /Dv,a)^3 is constant during dilution, which corresponds to the extremely(?) inhomogeneous mixing scenario.

(Dv /Dv,a)^3 increases with decreasing (nd/nd,a), and for the most diluted parts of the cloud even exceeds the adiabatic value(?).

A: sampled in the rather undiluted region B: more representative for the diluted regions of the leg.

a shift of the droplet size PDF to larger diameters, which is opposite to the effect observed in cases I and II

Fig. 5d

the values of (Dv /Dv,a)^3 > 1 in Fig. 5d correspond to

the mixing diagrams for cases I and II and cases III and IV differ significantly, although their microphysical characteristics (e.g., mean droplet number and diameter) are similar. cases I and II : the mixing is rather homogeneous, with small droplets in the most diluted cloud regions, whereas cases III and IV : it is extremely(?) inhomogeneous, and in case IV the largest droplets are found in the most diluted cloud parcels.

An apparent difference between the four clouds is their degree of dilution.

(cases I and II : younger and less diluted than cases III and IV)

To quantify the nature and degree of cloud dilution : Paluch diagram

What causes the difference in the mixing characteristics of the four cases?

Analysis of dilution-Paluch diagrams

case I : The source level of air ~ 960hPa (CB: 914hPa)

the highest positive velocities: high [θq; Q] values,

small negative velocities are found almost exclusively in more diluted regions(= more close to the saturation line), with lower [θq;Q] values.

-It shows that at least some regions of the cloud have risen adiabatically more than 500 m with very little dilution by the surrounding air.

914hPa

940hPa 960hPa

Paluch diagrams for cases III and IV (Figs. 8 and 9) indicate that these clouds these clouds are highly dilutedare highly diluted, with many values of [θq;Q] close to the saturation line (i.e., the clouds are close to complete evaporation). In these regions of high dilution, the vertical velocity is mainly negative; however, there are a few samples in case IV (Fig. 8 ?) where [θq;Q] is close to the saturation line (i.e., highly diluted), but with positive vertical velocities.

Paluch diagrams : cloud in case I is ‘‘younger’’ and significantly lessdiluted than the clouds in case III and IV

in these (high transition length scale) regions homogeneous mixing (Da << 1) is more likely to occur.

(higher local energy dissipation rate)

Turbulent cloud core Turbulent cloud core

summary A description of the mixing process by a single Damkohler number is not sufficient (because it is based on the assumption that mixing is determined by a single length scale)

1. Introduced a transition length scale that separates the inertial subrange into a range of length scales for which inhomogeneous mixing dominates and a range for which homogeneous mixing is prevalent.

A small transition length scale: mixing characteristics to appear primarily inhomogeneous,A large transition length scale : a high probability for homogeneous mixing to occur.

2. Second part of the paper, four case studies were presented and their mixing characteristics were analyzed

In clouds that were highly diluted and close to dissipation (more aged and diluted): mixing is inhomogeneous

Analysis of dilution-Paluch diagrams

914hPa

940hPa960hPa